The present invention relates to high-purity polycrystalline silicon used in large quantities as substrate materials of semiconductor devices, and particularly to a method of producing a rod-form high-purity polycrystalline silicon using a silane (SiH4) gas as a raw material.
A half-century has been already passed since high-purity silicon began to be used for semiconductor devices. During this period, the utilization field of high-purity silicon has expanded by being supported by the remarkable progress in semiconductor devices, and the demand thereof has greatly increased. Also, with the improvement in the performance of semiconductor devices, improvement in the quality of a silicon wafer, which is a substrate thereof, has been demanded, and further improvement in the quality of polycrystalline silicon, which is a raw material thereof, has been demanded.
High-purity polycrystalline silicon is melted, and after being remade into a single crystal, it is used for semiconductor devices. As a method for the production of single crystal silicon, there are a floating zone melting method (hereinafter called the FZ method) and a Czochralski method (hereinafter called the CZ method), and forms of polycrystalline silicon used for the methods are different from each other. In the FZ method, polycrystalline silicon produced in a rod form having a diameter of several tens mm or larger and a length of from 1 m to 2 m is used, while in the CZ method, nugget silicon formed by breaking the rod-form polycrystalline silicon is used.
In the FZ method, silicon is not melted in a quartz crucible as in the CZ method, but part of the rod-form silicon is melted in a band from, and the molten silicon is single-crystallized in a state of being supported by the own surface tension. Thus, the quality of the polycrystal that is used largely influences the single crystallization. Conventionally, the FZ method is carried out at least twice to single-crystallize the polycrystal as shown in, for example, Japanese Patent Application Publication No. 45-19562. Specifically, by at least one preliminary FZ method, defects during the growth of polycrystal are vanished, and in at least one subsequent second FZ method using a seed crystal, a single crystal is obtained.
As disclosed, for example, in Japanese Patent Application Publication No. 56-45852, at the beginning of the growth of polycrystal, that is, during the period of time in which the entire surface of the silicon core is covered with newly deposited silicon, the temperature of the silicon core is maintained to be lower by several tensxc2x0C. than the temperature at an ordinary thermal decomposition. According to this, a method of reducing defects near the interface between the silicon core surface and newly grown polycrystal has been tried.
However, the above prior art has the following disadvantages. In the production method disclosed in the aforementioned Japanese Patent Application Publication 45-19562, the FZ method is carried out at least twice. Thus, there arises the disadvantage of the production cost becomes very high because of the FZ apparatus cost, the FZ operation cost (expenses and labor), and the material loss for the polycrystal regulation occurring each time at carrying out the FZ method. Further, the rod-form polycrystalline silicon obtained by the production method disclosed in the aforementioned Japanese Patent Application Publication No. 56-45852 cannot achieve a satisfactory single crystal by carrying out the FZ method one time. As a result, it also has the disadvantage that the production cost becomes very high.
From the above, recently, a method of producing a polycrystalline rod capable of achieving single crystallization by carrying out the FZ method only one time has been strongly demanded.
In order to meet the above demand, the inventors have investigated in detail the various correlations between the defects of the polycrystalline silicon, and the single crystallization rate, the silicon compound to be used, the growth conditions of the silicon suitable for the compound, various kinds of cores and the like. As a result, the inventors have obtained the investigation results that the defects such as a defect in a whisker form occurring near the interface between a silicon core and newly grown polycrystalline silicon, the porosity and the like reduce the single crystallization rate, that the favorable cleanness of the core surface reduces the occurring ratio of the defects, that the occurrence of the defects differs greatly according to the kinds of substrates, and the like. Also, the inventors have reached the conclusion that it is impossible to reduce the aforementioned defects to almost zero level, which we demand, with the present technique of cleaning the silicon core surface and maintaining it in the industrial thermal decomposition reactor.
As explained thus far, the present invention is a production method achieved through the above process. Its object is to provide a method of producing a rod-form high-purity polycrystalline silicon capable of preventing defects occurring to a newly deposited silicon layer by using a silicon core having a specific silicon surface at the occasion of producing the polycrystalline silicon.
In order to attain the above object, a method of producing a rod-form high-purity silicon according to the present invention is a method of producing rod-form high-purity polycrystalline silicon depositing silicon on a rod-form silicon core by a thermal decomposition of a silane gas, comprising the steps of:
coating the rod-form silicon core to be used with a specific silicon layer by previously depositing any one kind of silicon layer o f an amorphous silicon layer and a polycrystalline silicon layer made up of fine particles of silicon with different crystal axes from one another on a surface of the rod-form silicon core to be used by vapor growth; and
depositing polycrystalline silicon by using the core coated with the specific silicon layer.
The above invention method (hereinafter called xe2x80x9cthe present invention methodxe2x80x9d) is the method obtained as a result of the microscopic analysis of the beginning of the thermal decomposition of a silane gas with use of the substrate having various different surfaces. Specifically, the method utilizes the property of the silicon layer newly deposited on the silicon core that the structure similar to the surface structure of the substrate used is first deposited during the initial deposition of several tens xcexcm, and thereafter a dendrite being the original polycrystalline structure is gradually deposited. When the surface of the substrate is coated with a specific silicon layer such as amorphous silicon, or polycrystalline silicon layer made up of fine particles of silicon with different crystal axes from one another, the occurrence of the defects due to abnormal growth was hardly observed. The aforementioned specific silicon layer with a thickness of several xcexcm or more provides the effect. Further, the result that the substrate coated with the specific silicon layer has fewer occurrences of defects, as the storage until the time just before its use becomes better.was obtained. The present invention is the method made by industrially developing the above results.
By carrying out the aforementioned present invention method, it becomes unnecessary to adopt special reaction conditions capable of causing a trouble in order to restrain the occurrence of the defects at the beginning of the polycrystalline silicon production by the thermal decomposition of silane gas. Consequently, extremely favorable rod-form high-purity silicon without a defect which is suitable for the FZ method under the best deposition conditions for polycrystalline silicon, can be obtained.
Further, the method of producing the rod-form high-purity polycrystalline silicon, the vapor growth, may be chemical vapor deposition (hereinafter called the CVD method) with use of any one kind of gas of gaseous silicon hydrides (SinH2n+2.n=1 ,2, . . . ) and a mixture of the same hydrides.
Regarding the implementation of the above CVD method, the high-frequency plasma CVD method is suitable as the met hod for industrially depositing amorphous silicon layer on a silicon core, and the thermal decomposition method is suitable as t he industrial deposition method of the polycrystalline silicon film made up of fine particles of silicon with different crystal axes from one another.
As a raw material for the CVD method, the gaseous silicon hydrides, the mixture of the same hydrides, or the gas prepared by diluting the hydrides with an inert gas or a hydrogen gas, is used for both the above methods. Of the gaseous silicon hydrides, the industrially available gases are silane gas and a disilane (Si2H6) gas. Accordingly, the experiment on an industrial scale was made with use of these gases. By carrying out this method explained above, the same effects as the above present invention method can be obtained.
Furthermore, in the method of producing the rod-form high-purity polycrystalline silicon, the vapor growth for depositing the specific silicon layer may be physical vapor deposition (hereinafter called the PVD method).
In the above method, the vacuum evaporation method, sputtering method, and the sublimation method are cited as the PVD method, and the vacuum evaporation method is an industrially suitable method. For the evaporation source in this case, the high-frequency (HF) heating method of rod-form silicon the high purity of which is insured with ease is suitable. By carrying out this method, the same effects as the aforementioned present invention method can be obtained.